1. Field of the Invention
The present invention relates to a leak diagnostic system of an evaporative emission control system for internal combustion engines, and specifically to a leak diagnostic system that makes a diagnosis on leakage for purge air and fuel vapor.
2. Description of the Prior Art
Each car now has an evaporative emission control system as one of automotive emission control systems. This is a system that captures or traps any fuel vapors coming from a fuel tank mainly when the engine is not running and prevents them from escaping into atmosphere. As is generally known, a typical evaporative emission control system for an internal combustion engine, has a carbon or charcoal canister filled with activated carbon or charcoal for temporarily storing, trapping or adsorbing fuel vapors emitted from a fuel tank when the engine is not running, and a purge control valve disposed in a purge line connecting an induction system with the canister. Generally, the action of clearing or removing the trapped fuel vapor from the canister is called "purging". Usually, when predetermined engine operating conditions are satisfied after the engine is started, the purge control valve is opened and thus engine vacuum is admitted to the canister. Thus, the engine vacuum draws fresh air up through the canister via an air port. The fresh air flowing through the interior of the canister, picks up these trapped fuel vapors, and removes the trapped fuel vapors from the canister, and thereafter the purge gas is burned in the combustion chamber. If there is a hole or a leak in the middle of a fluid-flow passage or a piping ranging from the fuel tank to the induction system or the intake manifold or the intake collector section, or in the presence of the canister purge line hose damaged or disconnected, fuel vapors could escape into the atmosphere. Therefore, diagnosis for fuel-vapor leakage is so important. Such fuel-vapor leak diagnostic devices have been disclosed in Japanese Patent Provisional Publication Nos. 7-139439, 7-189825, and 10-274107. The fundamental concept of fuel-vapor leak diagnosis as described in these Japanese Patent Provisional Publications, is as follows. First of all, both ends of the previously-noted piping are closed by a valve means to establish a closed space. Then, the closed space is shifted to a state (e.g., a decompressed state) that there is a pressure differential (e. g., a pressure drop) relative to atmospheric pressure, for example by way of introduction of vacuum (a negative pressure). Thereafter, the change in internal pressure in the closed space is monitored, and the presence or absence of fuel-vapor leakage is determined depending on the rate of change in internal pressure. Generally, to establish the closed space, a so-called drain cut valve is used for opening and closing the air port of the canister, in addition to a typical purge control valve. To monitor or detect the change in internal pressure in the closed space, a pressure sensor or a pressure gage is located in the piping. For leak diagnosis, the Japanese Patent Provisional Publication No. 7-139439 teaches the comparison of the time rate of pressure-rise in the closed space, obtained with the purge control valve closed and the drain cut valve closed, with a predetermined threshold value. In the diagnostic system as disclosed in the Japanese Patent Provisional Publication Nos. 7-189825 or 10-274107, a leak area is arithmetically calculated or estimated on the basis of four parameters, namely a first parameter being an elapsed time DT3 measured from a time when the decompressing operation is started to a time when a predetermined pressure-drop p2 is reached, a second parameter being a pressure differential DP3 between an initial pressure value P.sub.0 of the closed space and an internal pressure P, sampled with the lapse of a predetermined time period t5 during which gas fluid-flow stops and thus there is no pressure loss after completion of the decompressing operation, a third parameter being a pressure differential DP4 between the initial pressure value P.sub.0 and an internal pressure P, sampled at a time when a predetermined pressure rise p3 is reached from the sampling time of the pressure differential DP3, and a fourth parameter being a time interval DT4 measured from completion of the decompressing operation to the sampling time of the pressure differential DP4 (see FIG. 4). For leak diagnosis, both of the two Japanese Patent Provisional Publication Nos. 7-189825 and 10-274107 teach the comparison of the calculated leak area with a predetermined threshold value. The previously-noted three Japanese Patent Provisional Publications use a popular pressure sensor (or a popular pressure gage). This is a relative-pressure measuring instrument that senses a pressure value on the basis of atmospheric pressure (serving as a reference pressure level). For instance, such a relative-pressure measuring instrument measures the difference between internal and external pressure on its pressure-sensing element. Since the external pressure is almost always atmospheric pressure, the pressure gage reads the difference between a given pressure and the pressure of the atmosphere. That is, the pressure reading of the instrument is a "gage pressure".